1. Field of the Invention
The present invention pertains to the art of hybrid vehicle powertrains and, more specifically, modulating torque in a hybrid vehicle powertrain during a ratio change of the transmission that occurs during regenerative braking.
2. Discussion of the Prior Art
A hybrid vehicle powertrain typically includes an electric motor, such as a high voltage induction motor, wherein driving torque of an engine is supplemented with electric motor torque produced by the electric motor. The combined engine torque and electric motor torque is transferred to vehicle traction wheels through a multiple ratio power transmission mechanism. A wet clutch assembly may be included in the power flow path between a torque input element of the multiple-ratio power transmission mechanism and a crankshaft of the engine. An example of a hybrid electric vehicle powertrain of this type is disclosed in U.S. Pat. No. 6,585,066, which is assigned to the assignee of the present invention.
Attempts have been made to reduce power losses normally associated with torque converter automatic transmissions by adding an electric motor. A powertrain configuration of this type combines the performance of an internal combustion engine with the advantages of an electric motor that complements the speed and torque characteristics of the engine. The hybrid arrangement also permits the engine to be deactivated when the vehicle is at rest or disconnected from the power flow path of the powertrain as the electric motor supplies driving torque. Such a hybrid arrangement improves fuel economy while reducing undesirable exhaust gas emissions.
During a process commonly referred to as regenerative braking in a hybrid powertrain of this type, charging a high voltage battery during vehicle deceleration collects the kinetic energy stored in the moving vehicle. During regenerative braking, required braking torque is allocated between a set of friction brakes and the electric motor, which acts as a generator. The amount of braking torque required as the vehicle decelerates is apportioned in real time by a control system between the hydraulic, mechanical friction braking hardware and the electric powertrain regenerative braking. The apportionment of wheel braking torque between friction and regenerative braking is balanced through the deceleration process to achieve as much regeneration as possible to improve fuel economy.
In the case of a coasting downshift for a hybrid electric vehicle, the regenerative braking function coincides with the coast mode. In some hybrid electric vehicles, since the motor is located between the engine and the transmission, the coast downshift is done with a significant level of negative torque at the input to the transmission. This negative regenerative braking input torque is sometimes much higher than the negative input torque typically experienced in conventional powertrain vehicles with step ratio transmissions during coasting or braking deceleration. This operating condition differs from operating conditions found in conventional powertrains, where coasting downshifts are done with only a slight negative or positive torque at the transmission input. The negative torque in the hybrid powertrain will cause shift shock in a manner similar to that found in a power-on upshift in a conventional powertrain. For example, during a power-on upshift, the conventional transmission remains initially in the upshifted torque ratio and a torque ratio change takes place before speed ratio change. During the speed ratio change, there is no significant change in wheel torque. The length of the shift depends on the amount of torque that the engine is producing and the amount of the effective inertia mass connected to the engine which is felt by the driver as a shock. Shift quality may be improved by controlling transmission input torque such as by reducing transmission input torque during a power-on upshift by retarding the engine spark to reduce engine output torque. This improves both the durability of the on-coming friction element and the smoothness of the upshift event. Torque modulation using spark retardation will satisfy the timing and repeatability requirements to satisfy shift quality targets, but this wastes some energy during the shift, which can only reduce torque, not increase it. Torque modulation also can be accomplished by using a fuel cut-off to reduce engine torque, but restoring engine torque following a shift event often is not repeatable using fuel control. In a conventional powertrain using a hydrokinetic torque converter, a coast mode occurs whenever the accelerator pedal is off, both with and without braking. As the vehicle slows, a coasting downshift must be executed to keep the engine speed within the desired range. In the case of downshifts during regenerative braking, drivability problems result if shift shock is not addressed.
Prior solutions to this problem have addressed the idea of removing regenerative torque during shift events by switching from regeneration to friction braking and back again. However, such methods have the problem that the transfer to friction braking leads or lags the duration of the shift event and such solutions tend to require overly complicated control systems. Therefore there exists a need in the art for a system that can maintain good shift quality when performing a downshift between gear ratios during regenerative braking in a hybrid vehicle.